Fleas are tiny, wingless insects. The adult is an external parasite of people, dogs, cats, and wild animals, including rodents and birds. Responsible in centuries past for the plagues that killed millions of people, the flea is now a far less serious but still troubling health threat and nuisance to humans and animals.

Common types

Fleas are familiar to most people with a dog or cat. They are the small, flat and normally hidden insects whose quest for blood sets the animal to frantically snapping at its tail or scratching its ears. Cat (and dog) fleas are the most familiar of well over 1,800 species and subspecies. A few other species have been instrumental in spreading serous human diseases, such as bubonic plague. Others pass diseases onto animals and a few burrow into the skin of their hosts.

Fleas belong to the order Siphonaptera and are subdivided into three superfamilies. Pulicoidea, with about 25 genera, contains most species of medical and veterinary importance such as the cat, Oriental rat and sticktight fleas. Certopsylloidea is the largest superfamily with 150 genera that are mostly Neotropical in distribution in southern Mexico, Central and South America, and the West Indies.

The best known to North Americans is the cat flea (Ctenocephalides felis), which pursues cats and dogs. The very similar dog flea (C. canis) is far less common. These fleas jump onto their hosts and remain there.

Few other flea species are familiar, but the Oriental rat flea (Xenopsylla cheopis) which carries the causative bacterium for the plague, retains notoriety for several hundred cases occurring in wild rodents in the U.S. over the last several decades.


Adult fleas, which move by running or jumping, feed on the blood of the host. The eggs hatch into worm-like larvae that develop off the host and feed on detritus or the bloody excrement of the adults. The flea structure, form and life cycle show perfect adaptation to life on the host.

The Egg – Flea eggs are smooth and white. Most species lay two to six eggs per day and can lay hundreds over a lifetime. Eggs are normally laid in the host’s sleeping and resting areas. When laid on the host, as do cat and dog fleas, the eggs fall out of the host coat, often at the animal’s sleeping quarters. (Hatching occurs in a few days if humidity is above 70% and temperatures are between 65-80 degrees Fahrenheit.) The flea embryo uses a sharp spine on its head, called an egg burster, to cut and tumble out of the egg.

The larvae, blind, limbless worms with circlets of hairs around each inter-segmental division, have three molts. Normally intolerant of light, rarely seen but quite mobile, larvae feed on debris on the floor of the nest of their parents. Adult fleas supplement the debris under the sleeping animal with undigested fecal blood of adult fleas

The Pupa – The mature larva spins a whitish, ovoid cocoon with debris embedded in it. This resting stage allows a flea to survive for long periods until stimulated to hatch by the appearance of a host. Pupae will not survive if humidity is low (as low as 45% for the rat flea).

The Adult – The adult flea, normally 1.5 to 4 mm long, is wingless, flat and brown. The neck is short, as are the antennae, which fit into a protective groove on the head when not in use. The eyes are not well developed. Some species, such as Leptopsylla segnis, the house-mouse flea, lack eyes entirely. Many species have combs, arrays of stiff bristles to keep the flea from being pulled out of the host’s coat, and hind legs adapted for jumping. Two notable exceptions to the active, jumping flea are the sticktight fleas (Echidnaphaga gallinacea of chickens) and the sand flea, chigoe,or jigger (Tunga penetrans) which parasitizes man and animals. The females of these fleas burrow into the skin of the host and remain attached for life.

Adult fleas feed by piercing the host’s skin with their mouth parts and penetrating a capillary from which they suck blood, using one or more pumps to convey the blood to the gut. If undisturbed, feeding is complete in 2-10 minutes. Female fleas take up about twice as much blood as males.

Health issues

Human Health Impact

Historically, the tiny rat flea has greatly impacted civilization as carrier of the plague. The tropical or Oriental rat flea is the main carrier of the plague bacterium, Yersinia pestis. Typically feeding on the brown rat, Rattus rattus, and the black or roof rat, R. norvegicus, the rat flea also readily feeds on people.

Plague has probably afflicted humans since before the time of Christ. The Philistines are recorded as suffering from a disease with symptoms like those of the bubonic plague. The first pandemic of record was probably in the sixth century, beginning in Egypt. During the late Middle Ages, the Black Death laid waste to Europe in another pandemic. An estimated 25 million people died in the fourteenth century. From 1664 to 1666, 70,000 Londoners dies out of a population of 450,000. Civil disorder broke out. Terrified neighbors even put plague victims to death.

What made the disease so frightening was that its cause was unknown. Only in the late 19th and early 20th centuries did a few brave researchers establish that the rat flea had to be present to spread the disease. That discovery came during the third pandemic, which began in China’s Yunnan Province in the 1890s and spread to the West Coast of the U.S. and throughout the world. Deaths in the U.S. occurred in San Francisco, Los Angeles and cities farther east, such as New Orleans.

The disease is characterized by rapidly developing high fever, headache, prostration, fatigue and delirium. By the second day, lesions know as bubos (hence bubonic plague) appear in the groin and armpits. Mortality rates are high.

Cases of plague still occur in the U.S., primarily in wild rodents. In the western states, 334 cases were reported from 1970-1994.

Murine Typhus – Found worldwide, several species of flea, including Xenopsylla cheopis (the main vector), Nosopsylla fasciatus and Leptopsylla segnis carry murine typhus. Flea feces transmit murine typhus when the host human scratches his or her skin to alleviate itching caused by fleabites, allowing the pathogen to enter the body. While thousands of cases occurred each year early in the 20th century, the disease is rare in the U.S., occurring primarily in the South. Symptoms include sudden high fever, headache, nausea, coughing and a spotted rash.

Flea-Caused Infections – The female chigoe, jigger or sandflea bores into the skin, usually of the feet, causing extreme irritation. If Tunga penetrans is not removed, it can cause an infection, which may become gangrenous. Chigoes are found in tropical Americas and Africa and are most prevalent in people who walk barefoot.

Skin Irritation – Annoying to some, fleabites can cause serious irritation in sensitive individuals. Bird fleas may become a problem when construction disturbs bird nests in buildings. The human flea (Pulex irritans) can also cause irritation but is now much less frequently encountered in the U.S. than cat and dog fleas.

Tapeworm – Young children may be at risk if they play in areas where pet excrement is present and the cat or dog has the tapeworm Dipylidium caninum. When tapeworm eggs are defecated by a cat or dog, flea larvae may feed on the excrement and ingest the eggs. The tapeworm eggs hatch in the flea’s larval gut. When the flea completes its development, a cat or dog may ingest the adult flea during grooming or nipping. A child coming into close contact with a pet may ingest a tapeworm-infected adult flea from the pet.

Animal Health Impact

Cat and Dog Flea – The cat flea, which parasitizes both dogs and cats and can transmit a species of tapeworm to pets, is the main flea of concern to most North Americans. The less common dog flea is similar to the cat flea.

Adult cat and dog fleas remain on the host (rather than jumping on the animal only to feed), causing severe irritation and vigorous scratching. This can lead to severe coat loss and frequent visits to a veterinarian. Both flea species may transmit the tapeworm Dipylidium caninum. The tapeworm’s eggs are defecated by the cat or dog and larval fleas consume the feces.

Sticktight Flea – An important poultry pest in subtropical America, the sticktight flea remains attached to a chicken, causing ulcers in which flea eggs are laid, and in heavy infestations, anemia. The sticktight flea also will attack cats, dogs, horses and humans.

Myxomatosis – This flea-vectored disease of rabbits wiped out nearly the entire rabbit population of the United Kingdom in the 1950s, eliminating rabbit trapping as an income source for many and reducing the food source of raptorial birds and predatory animals. The disease has been deliberately introduced in Australia as a means of controlling huge infestations of rabbits.

Flea control

Sanitation, insecticides and common sense are keys to efforts to control fleas. Good sanitation measures are important in conjunction with appropriate flea control products used according to label directions on the host or in the host’s habitat.

Cat and Dog Fleas

Insecticidal pet shampoos and dips, flea collars and total release aerosols, and powders can control cat fleas. Control is best achieved by using several products concurrently so that the flea infestation is attacked both on and off the host. Dipping or shampooing coupled with fogging the home and fitting the treated pets with flea collars is advisable.

Normally, no single product is sufficient to control an infestation of cat fleas. For example, treatment of the pet only will not combat fleas present in immature stages in the carpet. The immature stages will emerge as adults and cause reinfestations. Thus a combination of a pet treatment and a house treatment, including the area where the pet sleeps, is usually needed so the cycle can be prevented from recurring.

Pet Shampoos and Dips – A veterinarian, professional groomer or the pet owner can apply these products according to label directions. Shampoos are applied and rinsed off after a short time. Dips are ‘leave on’ products.

Pet Flea Collars – Flea collars are impregnated plastic strips that allow the slow release of the active ingredient. Collars rely on migration of the active ingredient over the coat of the pet, possibly aided by grooming, to reach the fleas.

Total Release Aerosols – Foggers are designed to fill a room with fine particles that settle on exposed surfaces and can penetrate to hidden interior surfaces. Fleas attacking the pet are often also found on the floor, especially in carpet and places where the pet sleeps and rests. Use of a total release aerosol is a convenient means of treating flea infested rooms uniformly and completely. Of course, people and pets should not be in the room during fogging. Be sure to read and follow the directions.

In recent years insect growth regulators have been added to foggers and to other flea control products. These materials interrupt the flea’s life cycle before adult fleas emerge to become a nuisance. Because IGRs are slow acting and do not affect the adult fleas, they are normally coupled with a conventional insecticide so that adult fleas can be controlled immediately.

Direct Aerosol Sprays – These aerosol products have valves that allow the can to be used in an inverted position so floor areas can be easily sprayed. They are useful for treating limited areas or where foggers might be inappropriate.

Powders – Powders applied to the pet are the dry equivalent to dips in that the powder is left on the animal. They also are easier to apply than dips.

Systemics – A successful innovation in flea control has been an orally or dermally administered insect growth regulator in pill form for dogs and liquid or gel for cats. Available only from veterinarians, the IGR is administered once a month, preventing eggs from hatching and breaking the life cycle. Since a systemic has no effect on other life stages, including adults, control is not immediate. A conventional adulticide treatment is needed before the IGR can control an infestation.

Miscellaneous – Traps with small lights aim at luring fleas to adhesive coated sheets to which they are to become stuck. The impact of these types of traps on flea infestations is unknown.

Public Health Control – Control of fleas that may carry plague or other diseases is the responsibility of state and federal public health authorities, which routinely conduct surveys for plague, recording the incidence of plague antibodies in wild and domestic hosts. When surveys have indicated the need, dusting the burrows of rodent hosts with suitable insecticide dusts has controlled potential plague-carrying fleas. The rodents themselves may be controlled by rodenticides.

People in western states should avoid contact with wild rodents because of the possibility of contracting plague. Backpackers and campers should be particularly careful. Use of an aerosol insecticide is a wise precaution against rodent fleas. Wood piles and similar cover for the rodents near dwellings should be removed.